Incorporation of aluminum into titanium nitride (TiN) coatings is known to enhance the high temperature stability of such coatings. TiN, for example, begins oxidation at about 500° C. forming rutile TiO2, thereby promoting rapid coating deterioration. Aluminum can slow degradative oxidation of a TiN coating by forming a protective aluminum-rich oxide film at the coating surface.
While providing enhancement to high temperature stability, aluminum can also induce structural changes in a TiN coating having a negative impact on coating performance. Increasing amounts of aluminum incorporated into a TiN coating can induce growth of hexagonal close packed (hcp) aluminum nitride (AlN) phase, altering the crystalline structure of the coating from single phase cubic to a mixture of cubic and hexagonal phases. Aluminum content in excess of 70 atomic percent further alters the crystalline structure of the AlTiN layer to single phase hcp. Significant amounts of hexagonal phase can lead to a considerable reduction in hardness of AlTiN, resulting in premature coating failure or other undesirable performance characteristics. The inability to control hexagonal phase formation has obstructed full realization of the advantages offered by aluminum additions to TiN coatings.